prevent concurrent path selection table calculations

[KonradHoeffner]

@Yagnap: the redundant path display code did not work anymore (maybe I broke it?) so I had to disable this:

  if (path !== getPath()) {
    notyf.error("New path was selected while SPARQL query was run. Not displaying old path results now.");
    return;
  }

I think this doesn't catch the case where a user repeatedly selects the same path that is already selected, possibly due to insufficient user feedback.

We should fix and improve this in the following way:

• When a user selects a path (or selects a target and there is only one path) and the table is being calculated for display, there should be a progress indicator and all other path selections should be temporarily disabled.
• When a path is selected that is already active, we need to detect this and not calculate everything again. Maybe show a green notification to the user "path already active".

[KonradHoeffner]

This does not work for me even after CTRL+F5 and npm install.

[KonradHoeffner]

P.S.: Ah, it covers the case where the user selects the same source again, but not when just the target is selected again, I will fix this. It also doesn't prevent the repeated SPARQL query on path selection.

[KonradHoeffner]

• [x] prevent recalculation of paths when the same target is selected
• [x] prevent recalculation of the table for the currently shown path

[KonradHoeffner]

The current code also just notifies but doesn't actually stop the recalcuation :-)